This invention relates to instrumentation for measuring the moisture content of material, particularly moving web material by using a transient thermal pulse.
In many industries, such as in the production of paper and textiles, drying of water in a web is part of the process. Due to the large heat of vaporization required for drying of water, large quantities of energy are consumed. Recent studies indicate that this is an area in which significant energy conservation can be achieved if the efficiency of the drying process can be optimized by precise measurement and control.
Currently available commercial instruments have a number of disadvantages which limit their widespread use. Most are too expensive, particularly for small business establishments having limited capital. This is particularly true of some instrumentation techniques, such as infra-red, neutron and microwave moisture sensing. They involve investments of $5000 to $20,000, or more.
Another disadvantage is the lack of an automated readout which would make possible feedback control of the process. The popular dew point instruments provide only temperature readings. Conversion to relative humidity requires an operator to make reference to a psychrometric table. In addition these instruments cannot be used for automated process control because no convenient electrical signal is produced which would be used for the purpose. This fault is also present in the class of moisture sensors which operate on mechanical principles.
Most of the available moisture sensing instruments, such as the hygroscopic chemical-resistance type sensors, do not have a linear response. This disadvantage necessitates the use of sophisticated electronics to compensate for the nonlinearity. Another limitation is slow response. It is not uncommon for moisture sensing systems to require stabilization times on the order of more than 30 seconds to several minutes.
Yet another limitation is the size of the head of available instruments which contain the probe. Large probe heads are inconvenient for installation due to the large size of the instrumentation package.
Instruments requiring air sampling for reference are also inconvenient. The lack of environmental stability is another disadvantage. Hygroscopic chemical and ion-exchange-resin type resistance probes usually deteriorate with time when exposed to contaminants, such as smoke and certain chemicals. This affects calibration which leads to inaccurate control. Calibration is difficult with many instruments.
Periodic recalibration is essential to insure repeatable results. Only a small number of instruments make provision for this, and in most cases the instrument probe must be dismounted and placed in a humidity controlled chamber for comparison with a standard.
The disadvantages are largely responsible for the limited use of moisture sensing systems in processing. Any instrumentation which minimizes or eliminates such disadvantages would therefore be of great interest.